Utilizing an altitude sensor to control fan speed

ABSTRACT

A heat-generating device is characterized to determine the relationship between the speed of a DC cooling fan, and the thermal margin of the heat-generating device. Since the speed of DC fans is substantially linear with respect to their input voltage, the speed of the fan may be adjusted within a system to provide the speed necessary for cooling needs. An altitude is input to a converter which uses the characterization of the heat-generating device to determine a fan speed necessary at that altitude to cool the heat-generating device to a temperature within its operating range. The converter then controls the voltage supplied to the DC fan to result in the needed fan speed.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is related to application Ser. No. ______ entitled, “Technique for Sensing Altitude from Fan Speed,” filed on or about the same date as the present application, and hereby incorporated herein by reference. Application Ser. No. ______ discloses and claims a technique utilizing the altitude calculated from the fan speed in a method to set a fan speed sufficient to allow for proper processor thermal margin.

FIELD OF THE INVENTION

The present invention relates generally to the field of cooling technologies and more specifically to the field of cooling technologies within a device enclosure where cooling efficiency is related to fan speed and altitude.

BACKGROUND OF THE INVENTION

As altitude above sea level increases, atmospheric density decreases. This decrease in atmospheric density is responsible for a reduction in cooling capacity of a fan running at a given speed. Since there is less air at higher altitudes, at a given fan speed fewer air molecules will be passing over a heat-generating device, than would be present in the identical system at a lower altitude. This fact presents a problem for designers looking to characterize system requirements, since a given configuration that works well at sea level, may be sufficiently degraded in cooling capacity at higher altitudes such that some electronic devices may no longer be operating within their thermal design margins.

Designers have typically solved this problem by requiring sufficient cooling of all of their systems for performance at altitude. However, this solution is not optimum for systems operating at sea level, since the same system could operate at a higher frequency at sea level due to the improved air-cooling present at sea level. System performance could be maintained at all altitudes by requiring fans in high altitude systems to run faster, however this requires knowledge of altitude. While it is certainly possible to require users to input altitude information upon first use of a system, this approach is prone to errors. There is a need in the art for a method allowing electronic systems to detect their operating altitude so that they may respond accordingly.

SUMMARY OF THE INVENTION

A heat-generating device is characterized to determine the relationship between the speed of a DC cooling fan, and the thermal margin of the heat-generating device. Since the speed of DC fans is substantially linear with respect to their input voltage, the speed of the fan may be adjusted within a system to provide the speed necessary for cooling needs. An altitude is input to a converter which uses the characterization of the heat-generating device to determine a fan speed necessary at that altitude to cool the heat-generating device to a temperature within its operating range. The converter then controls the voltage supplied to the DC fan to result in the needed fan speed.

Other aspects and advantages of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view of a DC fan and converter according to the present invention.

FIG. 2 is a graph showing the relationship between fan rotational speed and altitude in an example embodiment of the present invention.

FIG. 3 is a graph showing the relationship between fan rotational speed and processor thermal margin in an example embodiment of the present invention.

FIG. 4 is a flowchart of an example embodiment of the control of a fan using an altitude according to the present invention.

DETAILED DESCRIPTION

FIG. 1 is a view of a DC fan and converter according to the present invention. In an example embodiment of the present invention a DC fan 100 including fan blades 104, a motor 102, and an electrical port 108 is provided to cool a heat-generating device. Optionally, the DC fan 100 may have the ability to output its rotational speed from the fan 100 itself without any additional devices. Alternatively a speed sensor 106, such as an opto-electronic device that counts fan blades 104 may be used for DC fans 100 without the ability to output their rotational speed. Also optionally, the speed data from the fan 110, or the speed data from the speed sensor 112 is then input to a converter 114. The converter 114 also receives an altitude input 118 from the user or another source. Note that within the scope of the present invention this altitude may come from a wide variety of sources. The converter 114 is programmed using data obtained by characterizing the thermal margin of a heat-generating device with respect to fan speed, along with the relationship between altitude and fan speed necessary for a given amount of cooling. The converter 114 then sends a control signal 116 to the fan to achieve the required fan speed. This control signal 116 may digitally set the fan speed, or it may simply adjust the DC voltage input to the fan. Optionally, the converter 114 may monitor fan speed itself and adjust the control signal 116, as needed until the fan speed data it receives shows that the fan is operating at the required fan speed. While FIG. 1 shows a discrete converter device 114 for simplicity and clarity, other embodiments of the present invention may include the converter function in other electronic devices present in the overall device that is being cooled by the DC fan 100. For example, in a computer system cooled by the DC fan 100, the converter functionality may be built in to the processor chip, or may operate in software under the computer operating system. The physical location and construction of the converter 114 is not critical to the present invention, and the converter 114 functionality may be implemented anywhere desired by the system engineer. A sample of DC fan characterization data is shown in FIG. 2.

FIG. 2 is a graph showing the relationship between fan rotational speed and altitude in an example embodiment of the present invention. Since the atmosphere is less dense at altitude than at sea level, a DC fan 100 supplied with a constant power voltage will rotate at a higher rate at higher altitudes. An example graph of this relationship between rotational speed and altitude is shown in FIG. 2. In this example graph of a characterization of a DC fan 100, the horizontal axis 204 represents altitude above sea level, measured in feet, and the vertical axis 202 represents rotational fan speed, measured in revolutions per minute (RPM). In this example embodiment, the characterization data 200 is represented by a straight line. Naturally, most embodiments of the present invention will take fan speed data at a variety of atmospheric pressures related to a variety of altitudes and then a curve will be fit to the data. This curve may be linear in some cases, but other curves may be fit to the characterization data within the scope of the present invention.

Note that in this example characterization graph, at a first data point 214, the DC fan 100 rotates at 2500 RPM (represented by point 206 in FIG. 2), and at a second data point 216, when the DC fan 100 is at an altitude of 2000 feet (represented by point 208 in FIG. 2). At a higher altitude of 12,000 feet (represented by point 212 in FIG. 2), the DC fan rotates at 3000 RPM (represented by point 210 in FIG. 2). While this sample characterization data is linear, characterization of other DC fans 100 may result in non-linear characterization data within the scope of the present invention. This characterization data may be described by an arithmetic algorithm, a look up table, or other equivalent mechanisms or methods for calculation of an altitude when given a fan rotational speed. The resulting characterization data is then programmed into the converter 114 shown in FIG. 1.

FIG. 3 is a graph showing the relationship between fan rotational speed and processor thermal margin in an example embodiment of the present invention. As fan speed increases, the amount of air flowing over a heat-generating device also increases. This increased airflow results in more efficient cooling of the heat-generating device resulting in a lower temperature of the heat-generating device. This relationship is shown graphically in FIG. 3. In this example graph of the relationship between the temperature of a heat-generating device, the horizontal axis 304 represents the fan speed, measured in RPM, and the vertical axis 302 represents the temperature of the heat-generating device, shown as thermal margin in a processor, and measured in degrees Centigrade (degrees C). Processor thermal margin is the temperature difference between the current temperature of the processor and the maximum allowed temperature. Lower actual temperatures of the heat-generating device result in larger thermal margins. The example thermal data 300 shown in FIG. 3 is represented by a straight line, however other embodiments of the present invention may result in non-linear thermal data.

Note that in this example thermal graph, at a first data point 314, at a fan speed of 2500 RPM (represented by point 308 in FIG. 3), the processor has a thermal margin of 1 degree C (represented by point 306 in FIG. 3), and at a second data point 316, at a fan speed of 3000 RPM (represented by point 312 in FIG. 3), the processor has a thermal margin of 8 degrees C (represented by point 310 in FIG. 3). Thus, for an increase in fan speed of 500 RPM the processor thermal margin increased by 7 degrees C, which may be critical to processor performance in some designs.

FIG. 4 is a flowchart of an example embodiment of the control of a fan using an altitude according to the present invention. In an example embodiment of the present invention, a method of setting a fan speed from a given altitude is begun at a start step 400. In a preliminary step 402 a heat-generating device, or a group of heat-generating devices, is characterized to determine their response to altitude as measured by rotational fan speed at a constant input voltage. Note that in some embodiments of the present invention, it may not be necessary to characterize every individual heat-generating device. Process variations within a given model of device may be sufficiently small that characterization of a sample of devices from that given model may be sufficient to generate characterization data usable by all devices of that model. In an optional step 404, a DC fan speed of a fan is detected. In a step 405 an altitude is received. In an optional step 407, the altitude is converted into a required fan speed. In a step 406, thermal margin of the heat-generating device is calculated from fan speed and an altitude. This thermal margin may then be converted into a desired fan speed, which is checked against the current fan speed in step 408. In a decision step 408, if the thermal margin is sufficient, the method ends at a finish step 410. If the thermal margin is not sufficient, the fan speed is adjusted in a step 412 and the method ends at a finish step 410. In some embodiments of the present invention, the method may follow an alternate path 414 and return to the step 404 where the fan speed is measured. Following this alternate path 414, the system may iteratively check and set the fan speed to keep the speed of the fan at the velocity necessary for adequate cooling of the heat generating device.

The foregoing description of the present invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed, and other modifications and variations may be possible in light of the above teachings. The embodiments were chosen and described in order to best explain the principles of the invention and its practical application to thereby enable others skilled in the art to best utilize the invention in various embodiments and various modifications as are suited to the particular use contemplated. It is intended that the appended claims be construed to include other alternative embodiments of the invention except insofar as limited by the prior art. 

1. A device, comprising: a fan configured to run at a variable speed; and a converter electrically coupled to said fan in such a way as to control a speed of said fan, wherein said converter receives an altitude and outputs a fan speed control signal calculated from said altitude to said fan.
 2. The device of claim 1, wherein said converter uses an arithmetic algorithm to calculate said fan speed control signal from said altitude.
 3. The device of claim 1, wherein said converter uses a look up table to calculate said fan speed control signal from said altitude.
 4. The device of claim 1, wherein said fan speed is controlled by said converter using a digital signal.
 5. The device of claim 1, wherein said fan speed is controlled by said converter using an analog signal.
 6. A device, comprising: a fan configured to run at a variable speed; a fan speed detector, outputting a fan speed; a converter, electrically coupled with said fan speed detector, wherein said converter receives said fan speed and an altitude and outputs a fan speed control signal calculated from said fan speed and said altitude to said fan.
 7. The device of claim 6, wherein said converter uses an arithmetic algorithm to convert said fan speed and said altitude to said fan speed control signal.
 8. The device of claim 6, wherein said converter uses a look up table to convert said fan speed and said altitude to said fan speed control signal.
 9. The device of claim 6, wherein said fan speed control signal is output by said converter as an analog signal.
 10. The device of claim 6, wherein said fan speed control signal is output by said converter as an analog signal.
 11. A method for the setting of a fan speed, comprising the steps of: a) characterizing a thermal margin of a heat-generating device with respect to a fan speed; b) receiving an altitude; c) converting said altitude into a required fan speed; and d) setting a fan to said required fan speed.
 12. The method of claim 11, wherein said converting step is performed using an arithmetic algorithm.
 13. The method of claim 11, wherein said converting step is performed using a look up table.
 14. A method for the setting of a fan speed, comprising the steps of: a) characterizing a thermal margin of a heat-generating device with respect to a fan speed; b) receiving an altitude; c) measuring a fan speed; d) converting said fan speed and said altitude into a required fan speed; and e) setting a fan to said required fan speed.
 15. The method of claim 14, wherein said measuring a fan speed step is performed by said fan.
 16. The method of claim 14, wherein said measuring a fan speed step is performed by an optoelectronic device.
 17. A device, comprising: means for receiving an altitude; and means for calculating a thermal margin of a heat-generating device from said altitude; and means for setting a fan speed corresponding to a desired thermal margin.
 18. The device of claim 16, further comprising: means for characterizing said thermal margin of a heat-generating device with respect to a fan speed. 